1. Field of the Invention
The present invention relates to an image forming apparatus having a charging device for charging developer remaining on an image bearing member such as a photosensitive member or a dielectric. Preferably, the present invention relates to an image forming apparatus having no dedicated cleaner. More specifically, the present invention is preferably applied to an image forming apparatus where, after developer (toner) is transferred onto a transfer material, a developing device cleans developer (toner) remaining on an image bearing member, and simultaneously, development is performed.
2. Description of the Related Art
Up to now, image forming apparatus using transfer type electrophotography such as copying machines, printers, and facsimile machines are known. An image forming apparatus is formed of a photosensitive member as an image bearing member which is generally a rotating drum, a charging device for uniformly charging the photosensitive member in a predetermined polarity and at a predetermined potential (charging process), an electrostatic latent image forming device, that is, an exposure device for forming an electrostatic latent image on the charged photosensitive member (exposing process), a developing device for visualizing by developer (toner) the electrostatic latent image formed on the photosensitive member (developing process), a transfer device for transferring the toner image from the photosensitive member onto a transfer material such as paper (transferring process), a cleaning device for removing toner remaining on the photosensitive member after the transferring process to clean the surface of the photosensitive member (cleaning process), a fixing device for fixing the toner image on the transfer material (fixing process), and the like. The photosensitive member is repeatedly used in image formation in the electrophotographic process (the charging process, the exposing process, the developing process, the transferring process, and the cleaning process).
Toner remaining on the photosensitive member after the toner image is transferred onto the transfer material is removed from the surface of the photosensitive member by the cleaning device. The removed toner is collected by and in the cleaning device. The collected toner accumulates in the cleaning device as waste toner. However, from the viewpoint of environment conservation, effective use of resources, and the like, it is preferable that no waste toner is generated.
Accordingly, image forming apparatus which return waste toner accumulated in the cleaning device to the developing device to reuse the toner in the developing device are known.
Cleaner-less image forming apparatus, which eliminate the cleaning device, are also known. A cleaner-less image forming apparatus removes toner remaining on a photosensitive member by the developing device after the transferrring process, and the removed toner is collected in the developing device to be reused in the developing device. In such a cleaner-less image forming apparatus, the developing device cleans up the toner remaining on the photosensitive member simultaneously with development of an electrostatic latent image (cleaning simultaneous with developing). xe2x80x9cCleaning simultaneous with developingxe2x80x9d is a method in which residual toner, that is, toner which is not transferred by the transfer device and is remaining on the photosensitive member is collected in the developing device in following developing processes. More specifically, the photosensitive member having residual toner thereon is then charged by a charging device, an electrostatic latent image is formed on the surface of the photosensitive member by an exposure device, and the electrostatic latent image is developed by the developing device. Simultaneously with the development of the electrostatic latent image by the developing device, toner remaining on the photosensitive member without being developed is collected in the developing device by bias for removing fog (potential difference Vback for removing fog as the potential difference between direct current voltage applied to the developing device and the surface potential of the photosensitive member).
In this method, the residual toner is collected in the developing device to be used in following developing processes. Therefore, no waste toner is generated, and troublesome maintenance for collecting waste toner is eased. Further, since the cleaning device is unnecessary, this method is also effective in miniaturizing the image forming apparatus.
However, if the charging device is a contact charging device which is in contact with the surface of the photosensitive member and charges the photosensitive member, when residual toner on the photosensitive member passes through a contact portion (charging portion) between the photosensitive member and the contact charging device, part of the residual toner which is charged oppositely to the normal polarity of toner (reversal toner) attaches to the contact charging device. Therefore, the contact charging device is contaminated by toner to an unacceptable level, and thus, the charging device can not sufficiently charge the photosensitive member.
Further, such reversal toner can not be appropriately collected in the developing device.
Therefore, the present inventors proposed an image forming apparatus of the cleaner-less type which, when the charging device is a contact charging device, is free from poor charging and a poor image by preventing residual toner from attaching to the charging device and by efficient collection by the developing device of the residual toner, and offers benefits of a cleaner-less image forming apparatus (JP 2001-215798 A).
The above-mentioned image forming apparatus includes a first developer charge amount controlling device (first developer charging member) for charging residual developer on a photosensitive member and located on a downstream of a transfer device in a photosensitive member rotational direction, and a second developer charge amount controlling device (second developer charging member) for charging residual developer on the photosensitive member and located on a downstream of the first developer charge amount controlling device and on an upstream of a charging device. The first developer charge amount controlling device charges residual developer, that is, developer which is not transferred by the transfer device and is remaining on the photosensitive member, in an opposite polarity to the normal polarity of the developer. Then, the second developer charge amount controlling device charges in the normal polarity the residual developer which was charged in the opposite polarity to the normal polarity. After that, the charging device charges the photosensitive member, and simultaneously, appropriately charges the residual developer.
Since the residual developer is prevented from attaching to the charging device and the developing device efficiently collects the residual developer, an image forming apparatus which is free from poor charging and a poor image and which offers benefits of a cleaner-less image forming apparatus can be provided.
However, when a developer charge amount controlling device is in contact with a photosensitive member, developer may remain more or less at the contact portion between the developer charge amount controlling device and the photosensitive member. At the instant when bias is applied to the developer charge amount controlling device, and at the instant when the bias application is broken, the residual developer no longer stays at the contact portion between the photosensitive member and the developer charge amount controlling device, and is transferred onto the photosensitive member, to thereby result in a poor image.
An object of the present invention is to provide an image forming apparatus which retains developer to be excessively expelled from a developer charging member onto an image bearing member.
Another object of the present invention is to provide an image forming apparatus which can make appropriate the charge amount of residual developer on an image bearing member.
A further object of the present invention is to provide an image forming apparatus capable of preventing a poor image due to residual developer on a developer charging member and maintaining high image quality for a long time.
Yet another object of the present invention is to provide an image forming apparatus which prevents residual developer on an image bearing member from attaching to a charging member.
A still further object of the present invention is to provide an image forming apparatus suitable for a cleaner-less system without a dedicated cleaner.
An additional object of the present invention is to provide an image forming apparatus, which improves the efficiency of collection of residual developer on an image bearing member to a developing device.